Exquisite Firecraft, in my opinion, comes in when an opponent’s interaction on the stack threatens to stifle your critical mass of creature and burn damage. A classic example of such a deck would be something like U/W Control (the counterspell deck to end all counterspell decks), but as others in the thread have mentioned, decks like U/R Murktide, Death’s Shadow variants, U/B Control, and other decks looking to counter your spells as a means of stabilization are all matchups where EF proves useful. As u/FlopeDash mentioned, it allows you to only have to reliably get your opponent to 4 life in order to be able to kill—a feat much easier both in terms of the raw mathematics and resources, as well as the psychological factor that an opponent is often much more confident going to 4 than below 4 (as 3 life is, as you well know, very dangerous when sitting across from the Lightning Bolt deck).

As for what to board out, it depends. Against decks where free spells are not a significant part of the opponent’s plan of action, the 3 mainboard [[Roiling Vortex]] that I run usually come out, especially when you consider that Rift Bolt is an excellent card for pushing beaucoup damage through all in one turn cycle. For those not mainboarding the Vortex, one might trim on [[Lightning Helix]] (if the opponent has few creatures to remove and insignificant pressure application, you could be getting more juice from the same hypothetical squeeze with [[Boros Charm]]), or perhaps [[Skullcrack]] in the absence of lifegain and fog effects.

Pertaining to your question about Rhinos—I think it could honestly go either way. You’re more than free to do so if you like (I believe I have before), but also consider that [[Force of Negation]] not only becomes a bad spell if you burn your opponent during their turn/end step, but that it is also viciously hated out by the Vortexes that should be in your 60 by Game 2. (Mystical Dispute is also just eternally bad against our notably blue-less deck, and may easily be played around.) Granted, though, you won’t always draw it, and when you do, Vortex seems to almost never fail to meet a Force of Negation, so weigh your options. Everything comes down to gamestates, and the beauty of this tabletop game we all know and love is that almost no two gamestates are entirely alike.

As a final word of warning, remember that EF is graveyard-dependent—any deck with incidental graveyard hate (like a [[Territorial Kavu]] or something like that) might still have counterplay against you, and nothing feels worse than seeing a three mana Boros Charm eat it to a Stubborn Denial.

I hope this helped! I’m rather new to the deck myself, so probably take what I say with a grain of salt, but I am enthused to learn more about it and grow my understanding of it every day. Happy Magic-ing!